The present invention pertains to the class of digital communication devices which receive messages according to an intermittent schedule known to the receiver and where power saving is important. Examples of devices belonging to this class are pagers, and battery-operated cellular phones operating in standby mode awaiting a page from a base station announcing an incoming call. A cellular phone may also receive short user information messages in the same way that pagers do.
During reception of the message in an active mode, the communication device is fully active and a relatively high timer clock frequency is required to support the digital processing of the device. Between scheduled message receptions the device typically enters a sleep mode during the inactive periods between message receptions. During sleep mode virtually all components of the communication device except for the system timer are powered down to minimize power consumption. The system timer is needed to maintain system time and terminate sleep mode for the next scheduled message reception. Because sleep mode typically has a very high duty cycle, it is important that the sleep timer itself have very low power consumption during sleep mode. Accordingly, prior art recognizes the virtue of low power timers for sleep mode. For example, U.S. Pat. No. 4,860,005-DeLuca et. al. (1989) describes the use of a real-time sleep timer programmed to turn on the receiver and a higher frequency timer to process the scheduled message. The oscillator for a real-time clock typically utilizes a low frequency 32.768 kilohertz crystal termed a watch crystal. Owing to the low frequency of the watch crystal oscillator timer consumption during sleep mode can be as low as 5 microamperes with present day integrated circuit technology. In contrast a separate, much higher frequency, reference oscillator is needed for the timer during active mode to support high speed timing digital processing for message reception. Typical battery consumption for present day reference oscillators in cellular phones is approximately 2 milliamperes.
In U.S. Pat. No. 5,613,235, Kivari et al., the importance of the synchronization factor in design for power savings and advances is pointed out to first generation cellular phones. First generation cellular phones are the analog AMPS standard and digital derivatives thereof. Kivari et al. provide for an apparatus for compensation of synchronization error by method of oscillator calibration. That is, the sleep mode oscillator is calibrated relative to the higher frequency accurate active mode oscillator to estimate the frequency error of the sleep oscillator. The cumulative sleep mode timing error is then removed at the beginning of active mode based on the estimated frequency error. The general methodology of using oscillator calibration to compensate the shortcomings of in inaccurate timer is however not new to the Kivari patent and has been applied in other applications, as can be seen, for example, in the reference, “Low Power Timekeeping”, M. Bloch, M. Meirs, J. Ho, J. R. Vig, and S. S. Schodowski, 1989 IEEE International Frequency Control Symposium.
While Kivari et al. assert the power saving advantages in the use of a low-frequency oscillator for the sleep timer, it is silent to the attendant drawbacks, specifically inherently poor frequency accuracy or stability. In general low frequency oscillators have a larger frequency error relative to higher frequency reference oscillators and consequently a large system timer error accrues during sleep mode. When the communication device switches to active mode to receive a scheduled message the accumulated sleep mode timing error represents synchronization error during message reception. When synchronization error exceeds a predetermined amount, the receiver is unable to demodulate the burst message and loss of synchronization is said to occur. Under condition of loss of synchronization, a process of reacquisition which involves the processing of extraneous transmissions from the message sources such as synchronization bursts is required before reception of the desired message is possible. Allowance for the overhead of reacquisition at the beginning of active mode requires that the receiver transition out of the low-power sleep mode and into the high-power active mode sooner. In this way sleep mode timing inaccuracy imposes a power consumption penalty in active mode. Generally, the problem of maintaining synchronization becomes more difficult as the symbol rate increases. This is because for a given loss-of-synchronization criterion expressed in message symbols the corresponding timing accuracy needed becomes smaller as the data rate increases. A recent worldwide trend toward higher data rates for communication devices such as cellular phones makes the loss of synchronization by the sleep timer an increasingly important factor with respect to power savings.
Kivari et al., while adequate specifically for first generation cellular phones, does not generally address the synchronization problem for the higher data rates of second generation cellular phones and beyond. Second generation cellular phones as known in the art are the present day phones as defined by standards bodies from the ground up as purely digital phones and the GSM worldwide standard is an example. Third generation phones as known in the art are now being defined by international standards bodies and are characterized by bandwidths that are much wider than is needed for voice communications to address future data communication needs. Shortcomings of the prior art of Kivari et. al. for current second generation phones and soon-to-appear third generation phones are as follows: (1) there is no provision for normal temperature changes and related oscillator recalibration management; (2) there is no provision for application of calibration compensation to the sleep mode timer during sleep mode; and (3) the sleep mode timer structure requires that the higher frequency reference oscillator be an integer multiple of the low frequency sleep oscillator.
The prior art does not maintain system timing accuracy during sleep mode. To see the shortcomings of prior art for current second generation cellular phones by way of numerical example, the GSM (Group Speciale Mobile) worldwide standard which has a symbol rate of 272 KHz and a maximum paging interval of 2.1 seconds. A typical GSM receiver will lose synchronization, i.e., be unable to demodulate a message burst, if the synchronization error exceeds four symbols. Accumulation of timing error less than four-symbols in the sleep or paging interval requires a sleep timer frequency error of better than 7 ppm (parts per million). In a practical design half that error or 3.5 ppm would be budgeted for the low-frequency oscillator to allow for other sources of synchronization error such as residual time tracking error at the time of the previous message. A typical low frequency oscillator such as a real time clock oscillator has a temperature dependent frequency excursion of approximately 84 ppm over the typical operating range of +/−35 degrees C. about room temperature That is, if the temperature of the cellular phone ranged from minimum to maximum operating temperature with a requirement of better than 3.5 ppm accuracy for the sleep timer, the sleep timer would need to be re-calibrated 84/3.5 or 24 times. Recalibration of the sleep timer requires continual active mode operation of the high-frequency high power consumption oscillator during calibration and typical calibration time for the GSM example is approximately 4 to 8 seconds. During standby mode for GSM the paging message bursts are only 4.6 milliseconds in duration, and the duty cycle for paging messages is as low as 0.0011 so that calibration incurs overhead in power consumption that should only be performed when necessary.
The preceding numerical example teaches the importance of management of the calibration of the sleep timer for power conservation. On the one hand calibration must be performed when oscillator frequency error becomes excessive or demodulation of the message will fail. But on the other hand unnecessary calibration incurs power waste by prolonging active mode just for calibration. The determination of when to calibrate in an operational environment when normal temperature changes can invalidate a prior oscillator calibration is critical to the successful application of the calibration methodology for compensating sleep oscillator frequency error. The invention of Kivari et al. does not address the critical issue of calibration management for a changing temperature environment.
The basic timer structure of Kivari et. al. has the drawback that compensation for sleep oscillator timing error accumulated during sleep mode cannot be applied until active mode because the reference oscillator must be powered up to apply the correction. A more useful sleep timer structure is one in which the sleep timer is continually compensated to account for the estimated frequency error of the sleep oscillator from oscillator calibration. In this way system time can be maintained accurately during sleep mode and used for accurate scheduling of power management even during sleep mode and for terminating sleep mode. For example, active mode cannot begin until the high-frequency oscillator for the active mode timer has been powered up in advance for settling time. The receiver synthesizer may also be powered up in advance of start of active mode to allow for settling time. Without such accurate scheduling the powering up of components in preparation for active mode must be started earlier to compensate for unknown sleep timer error accumulation.